1. Field of the Invention
This invention relates to a method for accurately and uniformly peening the inside wall of a conduit to a desired final peening intensity. It is particularly useful in rotopeening the inside walls of heat exchange tubes mounted in the tubesheets of nuclear steam generators in order to relieve tensile stresses in these tubes which might otherwise result in stress corrosion cracking.
2. Description of the Prior Art
Methods for peening the inside walls of metallic tubes are generally known in the prior art. Such methods are particularly useful in relieving or at least equilibrating the tensile stresses which may be induced across the wall of a metallic tube when that tube is radially expanded, as by a hydraulic mandrel or a cold-rolling tool. Such stress-causing expansions are routinely performed in the heat exchange tubes of nuclear steam generators, particularly in the sections of the tubes extending through the generator tubesheet, both during the manufacture and maintenance of the nuclear steam generator. Unfortunately, the resulting tensile stresses can lead to an undesirable phenomenon known as "stress corrosion cracking" in the tube walls if these stresses are not relieved. But in order to fully understand the dangers associated with such stress corrosion cracking, and the utility of the invention in preventing such cracking, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are comprised of three principal parts, including a secondary side and a tubesheet, as well as a primary side which circulates water heated from a nuclear reactor. The secondary side of the generator includes a plurality of U-shaped tubes, as well as an inlet for admitting a flow of feedwater. The inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in the tubesheet which hydraulically separates the primary side of the generator from the secondary side. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends. Hot, radioactive water flowing from the nuclear reactor is admitted into the section of the primary side containing all of the inlet ends of the U-shaped tubes. This hot, radioactive water flows through these inlets, up through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. The hot, radioactive water from the reactor transfers its heat through the walls of the U-shaped tubes to the non-radioactive feedwater flowing through the secondary side of the generator, thereby converting the feedwater to non-radioactive steam which in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
The walls of the heat exchange tubes in such nuclear steam generators can suffer a number of different forms of corrosion degradation, including denting, stress corrosion cracking, intragranular attack, and pitting. In situ examination of the tubes within these generators has revealed that most of this corrosion degradation occurs in what are known as the crevice regions of the generator. The principal crevice region for each of the U-shaped tubes is the annular space between the heat exchange tube and the bore in the tubesheet through which the tube extends. Corrosive sludge tends to collect within this crevice from the effects of gravity. Moreover, the relatively poor hydraulic circulation of the water in this region tends to maintain the sludge in this annular crevice, and to create localized "hot spots" in the tubes adjacent the sludge. The heat radiating from these "hot spots" acts as a powerful catalyst in causing the exterior walls of the heat exchange tubes to chemically combine with the corrosive chemicals in the sludge.
While most nuclear steam generators include blow-down systems for periodically sweeping the sludge out of the generator vessel, the sludges in the tubesheet crevice regions are not easily swept away by the hydraulic currents induced by such systems. Despite the fact that the heat exchange tubes of such generators are typically formed from corrosion-resistant Inconel stainless steel, the combination of the localized regions of heat and corrosive sludge can ultimately cause the walls of the heat exchange tubes to crack, and to leak radioactive water from the primary side into the secondary side of the generator, thereby radioactively contaminating the steam produced by the steam generator.
In order to prevent such corrosion and tube-cracking from occurring in the annular crevices surrounding the tubes in the tubesheet, various processes have been developed for radially expanding the sections of the tubes extending through the tubesheets so as to eliminate the annular space between the bores in the tubesheet and the heat exchange tubes. Such radial expansions may be implemented by hydraulic mandrels capable of applying fluid pressures of near 10,000 psi across selected sections of the tubes, or by cold-rolling tools which utilize pitched, tapered rollers capable of screwing into the open ends of the tubes, thereby widening them. However, such tube expansions create tensile stresses throughout the walls of the tubes in the tubesheet region which render them more susceptible to corrosion, thereby partially defeating the purpose of the tube expansion. Because the metal around the inner diameter of the tube was expanded a relatively greater amount than the metal forming the outer diameter of the tube, most of the tensile stress caused by the radial expantion was concentrated in the inner wall of the tube.
In order to relieve this tensile stress, shot peening processes were developed for hardening the inner walls of the expanded tubes. Such shot peening processes generally employed a nozzle which was slidably insertable through the open ends of the tubes in the tubesheet and which was capable of radially firing a large volume of tiny zirconia balls against the inner wall of the tube. The resulting high-velocity impingement of the hard, zirconia "shot" relieved much of the stress in the tube walls by compressibly work-hardening the inner wall of the tube. Since stress corrosion (and consequent cracking) seems to occur only in those regions of the tube walls which have undergone a threshold stress of between 10 and 15 kilo-pounds per square inch, the relief of the regions suffering maximum stress to a level less than 10 kilo-pounds substantially reduced the likelihood of stress corrosion.
Unfortunately, such prior art shot-peening processes are not without shortcomings. For example, if the motion of the peening nozzle along the longitudinal axis of the tube is not carefully controlled, a non-uniform peening pattern may result in the interior wall of the tube. Worse yet, if the peening nozzle should accidentally remain stationary for any significant amount of time during the shot-peening process, the high-velocity balls of zirconia can create new stress patterns in the walls of the tube which exceed the threshold stress limit for stress corrosion cracking to occur, and can even break completely through the tube walls, depending upon how long they remain stationary within the tube. Still other problems arise from the fragmented zirconia which becomes stuck in the inner walls of the tubes. Such fragments must be cleaned out of these tubes by means of a rotating, abrasive tool. This not only necessitates another time-consuming (and hence expensive) step in the maintenance procedure, but also creates a cloud of radioactive zirconia dust which may contaminate non-radioactive areas of the plant if this dust is not captured and disposed of properly. Additionally, the constant recirculation of the peening shot tends to change its peening characteristics, which in turn adversely affects the uniformity of the peening pattern created in the inner walls of the tubes even when the nozzle is moved at a uniform speed through the tube.
While rotopeening processes are known which are capable of peening and stress-relieving the inner walls of these tubes without the use of dust-creating zirconia shot, these methods likewise fall short of peening the inside walls of such tubes in a manner which is always uniform, consistent and accurate. If the rotopeening flappers rotate for too great or too short a time, they may either create new stresses in the tubes which will defeat the purpose of the operation, or fail to relieve the tensile stresses in the interior walls of the tubes. Additionally, the devices which implement such rotopeening methods lack any means to accurately control oscillating movements between the flappers and the inside walls of the tubes. The accurate control of such oscillatory movements is of paramount importance if the method is to uniformly rotopeen a given longitudinal section of a tube within a minimum amount of time.
Clearly, there is a need for a peening method capable of quickly and uniformly peening the inner walls of the heat exchange tubes mounted around the periphery of the tubesheet of a nuclear steam generator. Ideally, such a method should accurately and consistently peen the inner walls of such tubes to an intensity which effectively relieves the tensile stresses within these tubes without creating new stresses which could defeat the purpose of the operation, and without necessitating a separate abrasion step which creates a cloud of potentially contaminating radioactive dust.